٨Research Trends and Results

Concept and specific methods for management which
provides a flowcapacity margin
HATTORI Atsushi, HeadTAKEUCHI Yoshinori, Researcher; River Division, RiverDepartment
FUJITA Koichi, Research Coordinator for Watershed Management, River Department
ONUMA Katsuhiro, Senior Researcher
River Environment Division, Environment Department

Research Trends and Results(Key words) River course change, flow capacity, river course management, fine soil sedimentation, river narrowing,high water channel formation
1. Management of flow capacity anticipating rivercourse change
The enactment of basic policies for river
improvement and river improvement plans isaccompanied by the planning and implementation ofriver course excavation and tree cutting etc. to expand
flow capacity. When doing this, it is important tocarefully watch for change of river courses to study
the optimization of the management labor, but
sufficient knowledge is not always obtained. As one
method of ensuring the specified flow capacity in rivercourse sections prone to sedimentation, in addition to
the river cross-section necessary to transport a flood
flow, installing cross-section space for sedimentation(a margin) is also considered. This means that flow
capacity is managed by (1) as part of river course
design, predicting fluctuation of future flow capacity
according to sedimentation and, for example, setting
the optimum margin to balance the annual averagequantity of soil re-excavated and the maintenancelabor force, and (2) as part of river course
management, observing the state of sedimentation bysectional surveying to set judgment standards toreliably implement maintenance excavation before themargin is filled in by deposited sediment.
2. Incorporating the optimum margin in rivercourse designOn a river course with a gravel riverbed, if thelow-water channel is widened across a series ofsections, in areas where vegetation flourishes on slightelevations of the riverbed, every time fine sediment,
which is almost non-existent in the riverbed material,is deposited by a run-off, a high water channel forms,
returning the low-water channel to its original width1),and cases where this occurred in the relatively shortperiod of less than 10 years 2) are known. A rivercourse design which provides a margin was tested fora case where it is necessary to devise a way to quickly
reduce the size of the river section to maintain flow

capacity in this way. First, we constructed a simple
prediction model for high water channel reformation
radically simplified to a range where the essential
mechanism of fine soil deposition in flourishingvegetation areas is not harmed 2). This model wasapplied to the Sendaigawa River, confirming that themodel provides a certain degree of reproducibility of
the form and height of sedimentation, and that it can
predict and evaluate the river cross-section reduction.
This model can be used to study the form ofexcavation or margin of the river section, etc. whichslow the river section reduction rate to optimize themaintenance labor. 3. Inspection technologies to more reliably manage
flow capacityWe proposed the Basic River Course Management
Sheet as a tool to inspect flow capacity in cooperation
with the Kyushu Regional Development Bureau’sRiver Management Research Committee. It wasprepared by organizing data of various categories inorder to be able to longitudinally compare change overyears of the water level calculated for the time whenthe management target flow rate is flowing (twoversions: present situation and when the trees havebeen cut) and the average riverbed height. Sections
where, for example, the calculated water levelapproximates the design high water level (eliminating
leeway in the flow capacity) and the average riverbed
height has tended to rise in recent years, arereorganized so that they can be interpreted as sections
where there is danger of the flow rate being
insufficient. Using this sheet, it is possible to evaluate
the degree of reduction of the margin of the designedriver section in the present river course and to morereliably manage the flow capacity by, for example,
performing maintenance excavation before the margin
is filled by deposited sediment. We are counting on rational river course design andflow capacity maintenance being implemented in the٨Research Trends and Results

field in this way, further improving the level of rivercourse management technology.
[Sources]
1) Fujita et al.: Collected Reports of the Japan Societyof Civil Engineers, No. 551/II-37, pp. 47-62, 1996
2) Takeuchi et al.: Civil Engineering Journal, Vol. 52,No. 7, pp. 26-29, 2010